A conventional method for stabilizing oscillation frequency separation among a plurality of laser devices has been described on page 2-204 of "Part 2, National Conference Record, 1986, The Institute of Electronics and Communication Engineers of Japan". In the method for stabilizing an oscillation frequency separation among a plurality of laser devices, an oscillation frequency of a laser device selected from a plurality of laser devices is stabilized based on a resonant frequency of a sweep type Fabry Perot optical resonator. Oscillation frequencies of the remaining laser devices which are observed through the Fabry Perot optical resonator are stabilized to be provided with a predetermined separation based on the oscillation frequency of the selected laser device.
Another conventional method for stabilizing an oscillation frequency separation among a plurality of laser devices has been described on pages 61 to 64 of "Technical digest, Vol. III, 5th International Conference on Integrated Optics and Optical Fiber Communication, Oct. 1 to 4, 1985". In the method for stabilizing an oscillation frequency separation among a plurality of laser devices, a plurality of laser devices in which an oscillation frequency of a selected laser device is stabilized are driven to radiate output lights with respective frequencies, while an oscillation frequency sweep laser device is driven to radiate a sweep frequency output light in accordance with a sawtooth sweep signal. The output lights radiated from the plurality of laser devices are coupled with the sweep frequency output light radiated from the oscillation frequency sweep laser device to produce beat signals which are then converted to a train of pulse signals. The plurality of laser devices are controlled such that producing times of the pulse signals are of respective predetermined separations as compared to a producing time of a pulse signal which is produced on the basis of output light radiated from the selected laser device having a stabilized oscillation frequency. As a result, the plurality of laser devices are stabilized to radiate output lights among which oscillation frequencies are maintained to have a predetermined separation.
According to the former method for stabilizing an oscillation frequency separation among a plurality of laser devices, however, there are disadvantages that the number of laser devices which are controlled to be stabilized in a frequency separation is limited less than ten to twenty for the reason why the Fabry Perot optical resonator is used therein to detect an oscillation frequency separation so that the number of the laser devices to be controlled depends on a finesse of the Fabry Perot optical resonator, and that a mechanical stabilization is low because a mechanical sweep operation is inevitable therein.
According to the latter method for stabilizing an oscillation frequency separation among a plurality of laser devices, further, there are disadvantages that a frequency separation must be set therein in accordance with a frequency separation reference which is specified beforehand because a frequency separation reference signal is not produced therein, that a predetermined frequency separation is difficult to be maintained for a long time due to the time degradation etc. of the oscillation frequency sweeping laser device for the reason why the frequency separation reference is determined on the basis of a relation between the producing times of the pulse signals obtained from the aforementioned beat signals and voltages applied at the producing times to the plurality of laser devices, and that a target applying voltage must be set for a new laser device each time in a case where one of the laser devices is involved in a trouble and replaced by the new laser device.